RF systems, for example, Doppler Radar used for tracking weather systems, include power amplifiers to drive the signal sent from the antennae. Doppler Radar and other similar RF systems may send a pulsed RF signal. To generate the pulsed RF signal, a pulsed RF power amplifier must change power from a direct current (DC) power supply into the pulsed signal by changing the signal with RF transistors.
Amplification of low power signals is a cornerstone of many modern radio frequency devices. Examples include pulsed telemetry, radar, electronic countermeasures, and other applications including electronic warfare. Solid state power amplifiers used in RF applications can vary from single transistor designs to multiple combined transistors to meet specific power requirements of system designers.
Gallium Nitride devices are now the top choice for the power transistors due to their very high efficiency and long life. The typical structure of a transistor is a semi-conducting material located on substrate with a minimum, but not limited to, three connections to the electrical circuit. Two architectures commonly used in RF transistors are bipolar junction transistors (BJTs) and Field Effect Transistors or (FETs). The connections associated with BJTs are usually labeled as Emitter, Base, and Collector, while FETs label the circuit connections as Gate, Source, and Drain. Transistors used in amplifying circuits have outputs higher than the respective inputs. In current RF transistor designs, typically a negative voltage is applied to the Gate connection, the Source is generally connected to the ground or return, and the Drain is the higher voltage and higher current that powers the transistor.
Generally, the Drain side of the transistor is continuously powered, which creates heat, uses a large amount of power, and generates signal noise in the signal. The Gate function, or switch side in an RF device acts to limit current to a prescribed value to protect the device from self-destruction and additionally provides the low signal RF input. In an RF FET, the Gate side of the transistor also serves as the RF input while the drain is the output. Voltages applied to the Gate are referred to as a bias. The control voltages may be positive or negative with respect to the Source or return.
Semiconductor device manufactures publish specifications regarding the proper bias for FETs. An example of this specification would be a common RF FET having a Drain supply voltage of 50 volts DC and a Gate voltage of negative 2.7 volts that limits the current that is drawn on the Drain side of the transistor. A proper Gate voltage provides the best efficiency and the longest life.
Semiconductor designs can be optimized for operation in certain areas of the radio frequency spectrum. This design optimization may be both a critical requirement and a detriment. In a typical high power microwave amplifier, many stages of gain, or amplification, are required to produce a meaningful outcome signal. The multi-gain stage designs introduce significant noise into the system. During operation within a circuit, without any RF signal present, transistors create noise or oscillate within the desired band of operation and continue to draw operating current from the DC power supply while no signal is present for amplification (known as the inter-pulse period). Amplifier designers now rely on the integration of high power RF switches to address this noise.
These RF switches are very expensive and consume valuable space within the amplifier. In addition, the RF switches lack the power handling required. The other major undesired result of using the RF switches is the heat generated by the devices, which requires thermal management systems and techniques. A typical high-gain, high-power, solid-state amplifier can use large high-surface area heatsinks with fans to move the required volume of air needed to cool the RF switches. Some power solid state amplifiers may require complex and problematic liquid cooling systems consisting of pumps, tubing, and radiators. These cooling systems are large, heavy, and potentially complex, with reliability and maintenance issues.
The basic transistor structure above has several issues. As mentioned above, the Drain side of the transistor is powered continuously, which is problematic. The presence of this continuous power results in high heat, high current consumption, and unwanted noise—all significant and unavoidable problems.
Using the pulsed radar amplifier as an example, an approach might be to turn the transistor completely off during the non-pulse period, or intra-pulse period. Since pulsed signals have a short duration, e.g., in nanoseconds, the requirements of switching the transistor are challenging. Previous approaches have been to switch the gate side of the transistor between the optimum value for best efficiency and a full rail voltage to completely shut off the RF flow thru the transistor. Since the gate control voltages and current are much lower than the drain side, attempts to enhance performance with this method were unsuccessful. At the maximum gate bias the transistor continued to create noise. Switching the high voltage high current drain side effectively shuts down the transistor. A solution had to be realized using an ultra-fast DC switch that would perform at both the speed and high current required. Thus, there is still a need to create a switching solution for the above problems.
In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a letter that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.